The rat's tail is an extension of the vertebral column that
projects out the back side of the animal. The tail is a long cylinder
consisting of three concentric layers. The innermost core of the tail
is bone (vertebre). The bone is surrounded by a layer of tendons, and
the tendons are surrounded by a layer of skin. Blood vessels run the
length of the tail between the tendons.

The rat's tail has a thermoregulatory function: it serves as a
heat-loss organ. The tail is well suited for this purpose, because it
has no fur, has a large surface to volume ratio, and is perfused with
many blood vessels, especially at the tail tip and midlength (Yulong
et al. 1995). The rat's tail comprises only 5% of the rat's
surface area, but it can dissipate about 17% of the rat's body heat
(Monson and Oyama 1984).

Rats control their body temperature through their tails by
dilating or constricting their tail blood vessels. When the rat's
body temperature rises by a few degrees, the tail vessels (especially
the veins) swell -- called vasodilation -- permitting lots of
warm blood to flow through the tail (Vanhoutte et al. 2002).
The warm blood loses heat through the surface of the tail and returns
to the body at a lower temperature, thus cooling the rat. The warmer
the rat, the more blood flows through the tail, and the more heat is
lost (Thompson and Stevenson 1965, Little and Stoner 1968, Rand et
al. 1965, Raman et al. 1987).

When the rat's body temperature drops, the tail vessels shrink in
diameter -- called vasoconstriction -- thus restricting blood
flow to the tail (Owens et al. 2002). Less blood flows into
the tail for cooling, and body heat is conserved.

Cross section of the base of a rat's
tail at two different body temperatures (37º and
40º C). At 40º C, the diameter of the blood
vessels (especially the veins) is larger than at 37º C.
This larger diameter permits more blood to flow through the
tail: 15% more blood flows through the arteries and 125%
more flows through the veins at 40º C than at 37º
C. Adapted from Vanhoutte et al. 2002.

Balance

Rats use their tails for balance. They climb ropes and anchor
chains, they walk along fences and telephone wires and branches, and
their tails help them balance.

The rat's tail follows the same principle as the pole of the circus
performer on a high-wire. Look at the rat balanced on the rope at
right. Think of the rope as an axis, and the rat as able to rotate
around that rope. The rat's center of gravity must stay
directly above the rope -- if it moves to one side, gravity will
cause him to start rotating around the rope. If he does not correct
this rotation, he falls. The rat's tail increases the rotational
inertia of the rat, making it harder for him to rotate around the
rope (specifically, it increases his resistance to change in
rotational velocity). This rotational inertia gives him more time to
adjust his center of gravity back to the desired position. If the
rat's tail were magically removed, his rotational inertia would
dissappear, his body would easily rotate around the rope and he would
fall off or hang upside down by his feet.

The tail can also help change the center of gravity of the rat,
though this change will remain small because the tail weighs so
little. In the photograph to the right, the rat is holding on to the
rope with his hind legs, and his forequarters and tail are hanging
below and to either side of the rope. The rat's center of gravity is
low and close to the rope. A low center of gravity makes balancing
easier, because a larger rotation is required to tip the rat
over.

In the photograph to the left, a rat walks along a poster tube
suspended between two chairs. He holds his tail out to one side,
increasing his rotational inertia around the axis of the tube. This
gives him more time to make the tiny muscular adjustments he needs to
keep his center of gravity directly above the tube.

Note: I am a biologist, not a
rat breeder, therefore, this is not a section about
tailless rats as pets. This is an article about tailless rat
biology.

There are at least two mutations that cause taillessness in rats:
Tail-anomaly lethal (Tal) and stub (st).

 Tail-anomaly lethal (Tal): The autosomal
dominant mutation Tal is lethal in its homozygous state (when
the rat gets two copies of the gene): Tal/Tal embryos die
between days 9 and 10 of gestation (Hoshino et al. 1979). In
heterozygotes, (the rat has one Tal copy and one normal copy
of the gene), Tal causes taillessness.

Tal/tal heterozygoes vary widely: tail length ranges from
zero to normal, and the number of kinks in the tail ranges from one
to six per tail, and the kink severity ranges from barely perceptible
to complete hooks and corkscrews. Tailless rats also have lower body
weight than normal rats (Schaid et al 1982, Hoshino et
al 1979). This size difference between normal and tailless rats
is greater when taillessness came from the mother than the father,
possibly because the tailless mother's small size affects her ability
to rear her young. Some heterozygotes have normal tails yet can
produce tailless offspring, thus taillessness may skip generations
(Schaid et al 1982).

This variation in phenotype indicates incomplete penetrance
of the Tal mutation. Genetic penetrance -- the liklihood
that an individual carrying a mutation will develop the
characteristics caused by that mutation -- is estimated to be between
81.8% and 90.6% (Schaid et al. 1982).

 Stub (st): The second mutation known to
affect tail length is the recessive mutation stub. Homozygous
rats (st/st) usually die at birth (Ratcliffe and King, 1941),
indicating that stub's action is different from that of
Tal. The stub mutation is now believed to be exinct within the
population of laboratory rats (Schaid et al. 1982).

Tailless rats and thermoregulation

The rat's tail helps to regulate its body temperature. Tailless
rats do not lose heat as well as tailed rats -- their core body
temperatures rise higher and it takes longer for their high
temperatures to come down. For example, Spiers et al. (1982)
found that when tailless and tailed rats were warmed, the body
temperature of tailless rats rose 0.2-0.5 degrees higher than that of
tailed rats. Afterwards, it took the tailless rats nearly an hour
longer to cool down to a normal temperature than the tailed rats.